A handheld radiotelephone employs a speaker to convert electrical signals into sound waves in the human-audible frequency range of 20 Hertz (Hz) to 20,000 kiloHz (kHz), thus enabling a user of the phone to hear a representation of a caller's voice, as well as other sounds such as dial tones. The quality of the sound reproduction is an important factor in a customer's decision to buy a handheld radiotelephone. Audio quality of a speaker is determined by its frequency response over the audible frequency range. Manufacturers of speakers normally provide a specified frequency response of a speaker when it is used in a "free-field" environment. Speakers employed in radiotelephones, however, are rarely used in a free-field environment. Instead, the user places the radiotelephone against his ear to hear the sound, and the speaker is positioned very close to the human ear.
The goal of an acoustic engineer in the radiotelephone field is to choose the combination of speaker, enclosure, and preconditioning electrical circuitry that provides an acceptable audio quality. An acceptable audio quality is usually a measure of how flat, or variable, the frequency response of the speaker is in its environment over the frequency range of 300 Hz to 4 kHz. The flatter, or less variation, of the frequency response, the better the audio quality. A frequency response that is higher, or boosted, in the low frequencies than in the high frequencies will sound bassy and muffled; whereas a frequency response that is higher in the high frequencies than in the low frequencies will sound shrill and tinny.
Two types of speaker technology are in predominate use for handheld radiotelephones--piezo-electric speakers and dynamic speakers. Both speaker technologies are specified to work with an airtight seal between the speaker and the human ear. Achieving the airtight seal requires that the speaker have an airtight mount to the handheld radiotelephone housing and the radiotelephone housing have an airtight coupling where the housing meets the ear. The ear-housing coupling is achieved by forming a depression in the housing that conforms to the shape of the human ear. As long as the airtight seal is maintained, these types of speakers have been able to provide an acceptable frequency response.
As handheld radiotelephones become smaller in size, an airtight ear-housing coupling is difficult to achieve for all users because the housing is not large enough to form a depression in the housing surface that accommodates the size of all human ears. Consequently, for some users, an air leakage occurs between the housing and the human ear, resulting in a loss, or attenuation, of low frequency response. Tests on various speakers, including a Primo brand CR9 piezoelectric-type speaker have shown that this loss can average 15 decibels (dB) at 300 Hz, resulting in a tinny sound.
A low-acoustic impedance dynamic speaker is designed to boost the frequency response at the low end of the frequency range, and can be used to remedy the bass loss caused by air leakage between the housing and the human ear. An example of such a speaker is model no. HDR 0970 available Hosiden, Inc. Such a speaker--when mounted with an airtight seal between the speaker and the housing--however, exhibits excessive low frequency gain when an airtight seal is formed between the housing and the human ear. Consequently, the sound is bassy and muffled. Furthermore, there is a large variation of about 11 dB between the highest and lowest point in the frequency response, thus the audio quality is poor.
The size of the smaller handheld radiotelephones not only affects the seal between the housing and the human ear but also the size of the enclosure housing the speaker. Smaller handheld radiotelephones have less space to house the enclosure, and the size of the enclosure affects the frequency response of the speaker.
A need therefore exists for a speaker assembly for small handheld radiotelephones, which has limited space for enclosing the speaker and limited size for providing an airtight seal with the human ear, that provides acceptable audio-quality.